This paper by means of comparison discusses the existing issues in determining the productive ratio in farmland classification, and presents approaches to setting basic crops for various levels: (1) nationally, to select wheat, corn and rice as the basic crops based on their thermal ratio with rice thermal ratio 1; (2) provincially, to select basic crops according to the mature system with regard to their productive ratios between highest crop and specified ones. This paper, taking Shanghai as a case study, determines the productive ratio which can reflect the farmland classification quality and can be applied to forecast the standard crop production with a closer reaching to the practical output in regional farmlands.